1. Field of the Invention
The present invention relates to laser range finding systems and more particularly to the detection of return pulses utilizing properties inherent to a laser cavity undergoing relaxation oscillations.
2. Description of the Related Art
Laser range finders operate on the principle of measuring the time of flight of an intense, short duration pulse of energy from the time it is produced by a transmitter assembly to the time the reflected pulse from the downrange target is detected by a receiver assembly. Since the speed of light is a known constant, the time of flight of the pulse can be used to calculate the distance to the downrange target. Laser range finders typically consist of a collection of the following subassemblies: transmitter assembly, receiver assembly and controller assembly.
Presently, many implementations exist for a transmitter assembly to produce the desired high Intensity, short duration pulse of energy such as flashlamp pumping or Q-switching of the laser cavity. Beam forming and directing optics are used to focus the pulse on the downrange target. Characteristics of the transmitted pulse, such as temporal profile, spatial profile and wavelength, are preserved in the reflected pulse and may therefore be used to differentiate the reflected pulse from background or other interfering sources. The components of the transmitter assembly are often expensive, bulky and sensitive to misalignment. It would be desireable to eliminate many of these components while still retaining the functionality of the transmitter assembly.
The function of the receiver assembly is to collect the energy from the reflected pulse and detect its time of arrival. This is typically implemented using beam collecting optics to focus the incoming pulse on a photodetector such as a photomuluplier tube or a semiconductor photodiode. The reflected pulse from the downrange target is greatly attenuated due to such effects as atmospheric absorption and scatter, range to the target, diffuse scattering of the reflected pulse from the target and low reflectivity of the target. The peak intensity of the transmitted pulse must be great enough to insure detection of the attenuated return pulse by the receiver assembly under the most stressing conditions. The receiver assembly must also accommodate a wide dynamic range of reflected pulse intensities due to the fact that the intensity of the short time-of-flight return pulse from nearby targets is greater than the long time-of-flight pulses from distant targets. A desirable feature of the receiver assembly is the ability to increase the sensitivity of the receiver detector as a function of time-of-flight synchronized to the timing of the transmitted pulses.
The receiving assembly must also discriminate the return pulse from background interfering sources. The beam collection optics limits the field of view of the detector to the region illuminated by the transmitting assembly. This requires careful alignment of the receiver optics to the transmitter optics. It is more desirable to use the same optical system for both functions, however, the backscattering and retroreflections of the transmitted pulse from the optics may appear with great intensity at the receiver detector, which may result in saturation of the detector.
To further aid the receiver assembly in discriminating the return pulse, narrow band optical filters are used to reject signals that do not match the wavelength of the transmitted pulse. These filters can be costly and may require precise alignment. It would be desirable if the detector were inherently sensitive to only the same wavelength as the transmitted pulse.
The generation of short optical pulses with long repetition rates using electronic regeneration techniques in laser diodes is disclosed by Hung-Tser Lin and Yao-Huang Kao in their article entitled xe2x80x9cA Possible Way for Low-power Short Distance Optical Range Detector Using Regenerative Gain-Switched Laser Diodexe2x80x9d from the IEEE Lasers and Electro-Optics Society 1996 Annual Meeting Conference Proceedings. However, the pulse regeneration method described uses electronic means to sense the output pulse and modulate the power to the laser diode to induce oscillations. No direct optical feedback is employed in this method.
U.S. Pat No. 4,928,152, issued to Jean-Pierre Gerardin, discloses an apparatus in which the optical signal issued from a laser cavity is reflected by a target and re-injected Into the same laser cavity using the same collimating and focusing optics. The purpose of this configuration is to produce heterodyne beat signals as the CW laser diode is frequency modulated. This apparatus uses interferometery to determine distance, rather than measurement of the time-of-flight of an optical pulse.
U.S. Pat. No. 5,359,404, issued to Jeremy G. Dunne, discloses a laser rangefinder which determines the time-of-flight of an infrared laser pulse reflected from a downrange target. This apparatus is inherently sensitive to interfering signal sources and therefore requires additional means for the detection and discrimination of the return pulse. A digital logic-operated gate for the xe2x80x9copeningxe2x80x9dand xe2x80x9cclosingxe2x80x9d of a time window is required in the optical receiver for the purpose of rejecting interfering optical signal sources, such as internal reflections and atmospheric backscatter. Further filtering is provided by a narrow band interference filter tuned to the wavelength of the emitted laser pulse. Additionally, separate collimating and focusing optics are used in the transmitting and receiving portions of the apparatus.
It is a principal object of the present invention to provide the necessary functionality for a laser range finding apparatus while significantly reducing the number of components and subsequent cost and complexity by utilizing Inherent properties of a laser cavity.
It is another object of the invention to generate optical pulses, suitable for the purposes of range finding, by utilizing a laser cavity in a perturbed mode to induce relaxation oscillations.
It is another object of the invention to minimize the number of optical components by utilizing the same beam conditioning and directing optical assembly for both the transmission of the optical pulse and the collection of the return pulse from the downrange target
It is another object of the Invention to eliminate the high-gain photodetector amplifier electronics in the receiver assembly by optically amplifying the collected return pulse utilizing the gain medium of the same laser cavity used to produce the outgoing pulse.
The present invention is a laser range finding apparatus. In a broad aspect it includes an optical relaxation oscillator assembly, outcoupling optics, a photodetector and a controller. The optical relaxation oscillator assembly produces relaxation oscillations. The relaxation oscillations are a series of optical pulses having a controllable repetition rate. The outcoupling optics receives the series of optical pulses and redirects a minor portion of the energy of the series of optical pulses. A major portion of the energy of the series of optical pulses is adjusted in accordance with first desired beam propagation parameters. A photodetector receives the minor portion and converts the minor portion to an electrical signal representative of the series of optical pulses. A controller receives the electrical signal and determines the repetition period between the optical pulses. The controller provides a controller output to the optical relaxation oscillator assembly for adjusting the controllable repetition rate of the series of optical pulses produced by the optical relaxation oscillator assembly. During operation, the major portion of the energy of the series of optical pulses is directed to a reflecting target, reflected therefrom, collected by the outcoupling optics, and directed back to the optical relaxation oscillator assembly to stimulate subsequent relaxation oscillations, thus locking the period of the relaxations oscillations to the time of flight of the roundtrip path between the laser finding apparatus and the reflecting target.
The present invention eliminates the need for variable gain control of the receiving assembly detector by utilizing the Inherent time varying gain property of the relaxation oscillator. The relaxation oscillator provides variable optical amplification as a function of time, synchronized to the time of transmission of the outgoing optical pulse. The amplification is at a minimum after the generation and transmission of the outgoing pulse, thereby preventing amplification and detection of backscatter and retro reflections from the optical assembly. The optical amplification monotonically increases, thereby providing higher gain for typically less Intense, longer time-off-light return pulses from more distant targets.
The present invention eliminates the need for optical filters by utilizing the inherent narrow bandwidth amplification of the laser cavity to amplify only return pulses with a wavelength that is mode matched with the laser cavity. Since the same laser cavity is used for generation of the outgoing pulse, the return pulse is inherently mode matched with the laser cavity. This extremely narrow band amplification effectively filters out all out-of-band background noise and interference sources.
The present invention eliminates the requirement for a large dynamic range of the photodetector amplifier used to detect the return pulses. The return optical pulses are use to seed the subsequent relaxation oscillation in the laser cavity. The intensity of the optical pulse produced by the relaxation oscillation is independent of the intensity of the seed pulse. The photodetector detects the seeded pulses which are of uniform intensity.
Other objects, advantages and novel features will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.